This project deals with phenomena characterizing rodent populations in designed physical habitats as they reach optimum density and as crowding ensues with an increase to eight times this optimum. This year emphasis has been placed on two processes: (1) The increasing proportion of post-reproductive, aged animals accompanying increasing density. An hypothesis that older animals prey on younger ones, in the sense that they prevent the younger ones from fully developing behaviorally, was explored. That is to say, a single specie population can be examined from a predator-prey perspective. Too many predators (i.e., old animals) threatens the population with extinction. (2) Group differentiation within a population to produce a spatial structuring of the relations among groups so that the total pattern of social organization is one exhibiting bilateral symmetry with anterior dominance. This project includes parts of discontinued ZO1 MH 00824, and 28. BIBLOGRAPHIC REFERENCES: Koeppel, J.W., N.A. Slade, K. Harris, and R.S. Hoffmann. 1977. A Three-Dimensional Home Range Model. J. Mammal. 58: 213-220. Slade, N.A. 1977. Statistical Detection of Density Dependence from a Series of Sequential Censuses. Ecology. 58:--. In press.